The present invention relates to control circuits for pump motors. More specifically, the field of the invention is that of liquid level control circuits which automatically maintain the liquid level within a predetermined range.
In sump and water tanks, for example, the liquid level should be maintained within a predetermined range for proper functioning of the tank. Many prior art devices automatically control the liquid level within the tank by activating a pump when the liquid rises above a first predetermined level and deactivating the pump when the liquid level falls below a second predetermined level. Some of the prior art devices use mechanical or moving parts such as mechanical switches operated by rubber diaphragms, springs, rods, floats, or balls, all of which may tend to wear out or malfunction over time.
Other prior art devices use electrical or optical probes positioned within the tank to determine the liquid level and control the pump accordingly. For example, self-heating thermistors or conductivity probes may be used. However, such prior art systems using probes may be sensitive to humidity, moisture, changing temperatures, and varying voltage levels in the sensing circuit, all of which may produce erroneous results and subject the probes to wear. Also, contamination of the probes may adversely effect their performance. The probes and their associated circuitry may be adjusted to improve performance, but making the adjustments may be inconvenient and expensive.
What is needed is a liquid level control which reliably operates without the need for adjustment.
Also needed is a liquid level control which minimizes operating problems associated with contamination and mechanical wear.
A further need exists for a liquid level control which minimizes inaccuracies associated with varying temperatures.